The present invention pertains to methods and apparatus for routing optical signals, and more particularly to rapid switching of optical signals on the basis of frequency-selective interaction with an escort beam in a non-linear medium.
Fast optical switching is an essential component of high-speed optical networking. Optical switching methods currently employ either mechanical motion, such as the motion of mirrors driven by micro-electromechanical systems (MEMS) or, otherwise, are based upon modification of a material characteristic of the switching medium, which may be expressed in terms of the nonlinear polarizability of the medium. Thus, the effective index of refraction of the medium is modified, either by virtue of modifying the lattice structure of the medium or, as in the case of optical nonlinearity in semiconductor devices, by generation of real charge carriers.
A straightforward approach to switching based on nonlinear polarizability encompasses electro-optic devices, relying, for example, on the Pockels effect (or the Kerr effect) to induce (or modify) birefringence as a function of an applied electric field, thus changing the phase for certain polarizations. This is used in all electro-optic waveguide modulators. A salient disadvantage of such effects is that the induced phase modification will be nearly the same for all nearby wavelengths. Therefore, there is no simple way to make a wavelength-dependent switch. Moreover, to achieve a stable zero-phase configuration, electro-optic devices typically have “dither-and-feedback” circuits to hold quadrature points; however those both add to complexity and limit the extinction ratio.
While the nonlinearity of a switching medium may be naturally occurring, nonlinearities may also be achieved through synthesis of photonic crystals, as described, for example by Mazurenko et al., Ultrafast Optical Switching in Three-Dimensional Photonic Crystals, Phys. Rev. Lett., vol. 91, p. 213903, (2003), which is incorporated herein by reference.
Switching speeds are limited, in the case of real carrier generation, by recombination times which can be long (˜ns). Off-resonance excitation in the optical Stark effect regime (a splitting or shifting of energy levels of the material) is subpicosecond, however, typically requiring very high switching intensities, often >109 W-cm−2. Moreover, such fields give rise to generation of real carriers by one- or two-photon absorption, again implicating recombination time limitations mentioned above.
Limitations of the foregoing optical switching methods may be traced to their reliance upon the generally small third-order non-linear polarizabilities of materials, and the high absorption inherent in the fact that these third-order non-linear polarizabilities are achieved close to the semiconductor absorption-band edge.
It is thus desirable to implement a fast optical switch in a medium which is transparent and suitable for switching over a wavelength band sufficiently broad to accommodate a typical optical communications band, and in which optical switching may be based upon the typically stronger second-order non-linear polarizability of the material and is not limited by recombination times of free carriers in the medium.